1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device using the same.
2. Description of the Related Art
An exposure apparatus projects the pattern of an original (also called a reticle or mask) onto a substrate by a projection optical system to expose the substrate, using an excimer laser or a mercury lamp as an exposure light source. The substrate is coated with a photoresist, on which a latent image is formed by exposure.
The exposure process of exposing a substrate by the exposure apparatus requires precise exposure dose control on the substrate. A measurement device for measuring the illuminance of the exposure light that falls on the image plane (the plane on which a substrate is positioned) of the projection optical system is mounted on the substrate stage of the exposure apparatus.
Since the sensitivity of the measurement device is not constant under all circumstances involved, the measurement device must be calibrated in accordance with an appropriate schedule. In addition, since the device manufacturing line generally uses a plurality of exposure apparatuses, these apparatuses must have the same illuminance so that they have the same throughput. To meet these needs, a common calibration illuminance sensor can be used to calibrate the sensitivities of the measurement device in the plurality of exposure apparatuses.
In the sensitivity calibration using a calibration illuminance sensor, the illuminance sensor is positioned in the exposure region on the image plane of the projection optical system, and measures the illuminance. Also, the calibration target measurement device mounted on the substrate stage is positioned in the exposure region, and measures the illuminance. The sensitivity of the measurement device is calibrated based on the relationship between the outputs from the calibration illuminance sensor and the measurement device. Note that the sensitivity calibration of the measurement device typically means determining a calibration value, by which the output from the measurement device is multiplied, in order to correct the output from the measurement device.
An illuminance distribution formed in the exposure region typically has a trapezoidal shape. In other words, an illuminance distribution formed in the exposure region typically has a main region in which the illuminance is uniform, and peripheral regions located on the two sides of the main region. In the peripheral region, the illuminance lowers in accordance with the distance from the main region. Note that both the calibration illuminance sensor and the calibration target measurement device must be located in the main region upon sensitivity calibration. If one of the calibration illuminance sensor and the calibration target measurement device falls in either peripheral region, their outputs are naturally different from each other due to the difference in illuminance between the main and peripheral regions. This makes it impossible to correctly calibrate the sensitivity of the measurement device.
As the NA (Numerical Aperture) of the projection optical system increases, the width of the main region in which the illuminance is uniform narrows. Thus, it is becoming increasingly difficult to set the calibration illuminance sensor in the main region.
In general, the position of the substrate stage is controlled with a very high accuracy in order to precisely position the substrate. It is, therefore, easy to position the measurement device, mounted on the substrate stage, in the exposure region. It is also easy to position the calibration illuminance sensor in the main region, in which the illuminance is uniform, of the exposure region, as long as this sensor is mounted on the substrate stage as needed.
Unfortunately, when an improvement in the throughput is of prime importance, it is unfavorable to provide a holding unit, for holding the calibration illuminance sensor in the sensitivity calibration, to the substrate stage. This is because such a holding unit complicates the substrate stage, leading to an increase in its weight.
It is, therefore, desirable to adopt a scheme of setting the calibration illuminance sensor on the image plane of the projection optical system every time the sensitivity of the measurement device is calibrated. In this scheme, an arm can be attached to a driving mechanism that is independent of the substrate stage, on which the calibration illuminance sensor can be mounted. In this case, the calibration illuminance sensor can be positioned on the image plane by driving the arm by the driving mechanism.
The problem with this scheme, however, is that an error that arises upon attaching the arm to the driving mechanism or attaching the illuminance sensor on the arm degrades the positioning accuracy of the illuminance sensor. Furthermore, the attachment error directly turns into a positioning error, because a driving mechanism as described above generally does not cooperate with a position sensor, indispensable to precisely position the illuminance sensor as the positioning target object. Also, additionally providing a high-accuracy position sensor to the exposure apparatus increases the complexity of the exposure apparatus and the manufacturing cost.